Hi Circuiteers

I want to make a soft-starter for my tablesaw. I thought there'd be a simple solution online but actually it seems more difficult than I expected. I'm proficient with PICs so I'm leaning toward creating a PIC solution, along the lines of the below:

• Use the PIC ADC to sample rectified mains continually. This will determine the low point and record 2*mains frequency.
• The program will then turn on the triac a set time before the next anticipated mains low point.
• By steadily increasing this set time the motor will gradually power up (~5 secs)

I haven't included a circuit to avoid breaching the moderation rules. I just want to know if there's a better general approach to this. I'm hoping that my experience of PICs will make this much easier than some discrete solution

Robin

 

hi Robin,
I bought a device like this for electric heater control, works fine for motors.
https://www.amazon.co.uk/Yeeco-Cont...coding=UTF8&psc=1&refRID=V4BH1FSDRXS48357327J

E

 

hi Robin,
I bought a device like this for electric heater control, works fine for motors.
https://www.amazon.co.uk/Yeeco-Cont...coding=UTF8&psc=1&refRID=V4BH1FSDRXS48357327J

E

Looks good. Any idea why this is so much cheaper? It has the same power rating but doesn't mention inductive loads
https://www.amazon.co.uk/dp/B00HPTLZO4/ref=psdc_1938304031_t2_B0154I3ZUY

 

hi,
No idea, always big price variations with Amazon.

 

If it's a Universal motor, just use an ordinary light dimmer and increase the triac power rating,

which is what the Amazon controller looks like.

 

I want to make a soft-starter for my tablesaw. I thought there'd be a simple solution online but actually it seems more difficult than I expected. I'm proficient with PICs so I'm leaning toward creating a PIC solution, along the lines of the below:

• Use the PIC ADC to sample rectified mains continually. This will determine the low point and record 2*mains frequency.
• The program will then turn on the triac a set time before the next anticipated mains low point.
• By steadily increasing this set time the motor will gradually power up (~5 secs)

I haven't included a circuit to avoid breaching the moderation rules. I just want to know if there's a better general approach to this. I'm hoping that my experience of PICs will make this much easier than some discrete solution

Robin

Use a Pic with a random phase opto coupler MOC302x etc, use a simple zero crossing detect circuit as shown here in quite a few examples and then use a timer to sequentially increase the firing start point from 10% delayed phase angle back up to 100% firing of the Triac with a MOC3023 etc.
Max.

 

Use a Pic with a random phase opto coupler MOC302x etc, use a simple zero crossing detect circuit as shown here in quite a few examples and then use a timer to sequentially increase the firing start point from 10% delayed phase angle back up to 100% firing of the Triac with a MOC3023 etc.
Max.

Right yes. That's what I was getting at, with the addition of an opto-coupler. I need to weight up whether I'd want to add any other features possible with a PIC solution, otherwise I should just get the amazon unit.

 

A start.
Ignore the GP1 in, it was a different application.
Max.

 

Ok, I've decided to go down the PIC route so I can add a constant speed controller which detects the motor's rpm from the current ripple. I'm going to use a cheap mains-USB adapter (<£2) to drive the PIC. This circuit will sit on the motor side of a big stop-start switch which cuts live and neutral. On that basis, is there a good reason to use the opto-coupler? It'd be easier to just drive the TRIAC on the neutral/ground side via an emitter follower from the PIC. The stop-start switch is the sole means to cut power so I don't believe it necessary to interrupt the circuit on the live side.

Thoughts?

 

You really need to have an optocoupler.
DO NOT assume mains "polarity"!!!
Anything connected to switch mains is wise to have good isolation. You are hooking USB to this device. Get the wiring wrong, or move it somewhere else with a wiring error and you run the risk of The Big Bang.

As an example, years ago, a friend of mine was selling PCs and hooked up a system.
It went bang and killed the PC and the monitor. Just as well really as it could have been a customer.
This was caused by a couple of faults.
The first was the commercial molded power lead has the Earth and Neutral wire swapped. Yes, at the swittchboard, the Earth and Neutral are connected (here in Oz) and that in itself would not cause the damage, but then he used a home made extension lead that had the Active and Neutral swapped. This placed the Active on the monitor chassis. This went to Earth via the video lead and the PC.
Both devices were fried but after the smoke settled, and a calming cuppa, it was realizes how lucky he was to not have been killed.
Do not assume it will be wired corectly!
Make sure you have isolation between your mains switch and the controller. It does not cost much, but it you are worried about the extra expense, do not even start to make it.
The life you save my not just be your own.

 

I think you've misunderstood. The motor controller is only connected to the motor. The is no USB device here. If Neutral and Live are reversed then they're reversed for the motor and motor-controller simultaneously so it makes no difference there. The only problem will be if the 240VAC-5VDC converter is grounded to Live (and I've assumed it's floating or grounded to Neutral), but that would be a death-trap

PS. since found this on wikipedia. Interesting point about connecting the +ve rail to neutral to drive the TRIAC in Q2 & Q3 avoiding Q4

When mains voltage TRIACs are triggered by microcontrollers, optoisolators are frequently used; for example optotriacs can be used to control the gate current. Alternatively, where safety allows and electrical isolation of the controller isn't necessary, one of the microcontroller's power rails may be connected one of the mains supply. In these situations it is normal to connect the neutral terminal to the positive rail of the microcontroller's power supply, together with A1 of the triac, with A2 connected to the live. The TRIAC's gate can be connected through an opto-isolated transistor, and sometimes a resistor to the microcontroller, so that bringing the voltage down to the microcontroller's logic zero pulls enough current through the TRIAC's gate to trigger it. This ensures that the TRIAC is triggered in quadrants II and III and avoids quadrant IV where TRIACs are typically insensitive.[5]

 

ok.
The mains USB adapter is the power supply. I did misunderstand. That will work ok.
Just tha same, I'd use the opto driver, even if just to simplify firing the TRIAC. A number of my designs use optos as level shifters, and not for their isolation.It just makes things easier often.

 

The Fairchild AN-3006 shows methods for complete isolation of the logic circuit if needed.
For both detection and Triac firing.
Max.

 

Hello,

Mains isolation is needed when the user has to interact with parts on the circuit board that are connected to the circuit with materials that are conductive, or another device (such as a PC computer) that also connects to the line can create a ground loop. Otherwise isolation is not needed.

An example of where isolation is not needed is a regular triac dimmer. The dimmer is installed inside an electrical box and the only thing that the user gets to touch is the knob that turns it up and down. Everything is enclosed so the user cant touch anything, and if the user opens up the box then they are assuming they are an electrician who knows about the dangers of home level voltage levels (120vac to 240vac).

At the hobby level however it is often recommended to use isolation because the user may be playing around with the circuit and forget that it is connected to the 120vac to 240vac mains. You may ask what good this does though because the triac itself can not be isolated as it MUST be connected to the mains or it would not work unless of course you went with a power isolation transformer.

 

Ok this is good stuff. thanks a lot guys. You're right, isolation is easy and makes things a lot safer. Is there an alternative to a triac to avoid the 1.2V drop? I was thinking 2 mosfets back to back?

 

You could add a small correctly rated relay to shunt the triac when in normal run operation.
This would easily be detected and turned on with the Micro.
If doing so it may be wise to include a motor E-stop P.B.
Max.

 

The simplest soft-start solution is to use 200 feet of skinny extension cord. Not the best, certainly, but the cheapest and the simpleast

 

That is a great idea. At no load when the PIC is regulating the speed (say ~70% of no-load full power speed) the current will be low and the TRIAC will not dissipate much. Under load when full power is required the relay will kick-in and TRIAC dissipation will be zero. This will allow me to save on a big heatsink and down rate the TRIAC. I just need to worry about the moment just before full power is required and current is moderate

 

That is a great idea. At no load when the PIC is regulating the speed (say ~70% of no-load full power speed) the current will be low and the TRIAC will not dissipate much. Under load when full power is required the relay will kick-in and TRIAC dissipation will be zero. This will allow me to save on a big heatsink and down rate the TRIAC. I just need to worry about the moment just before full power is required and current is moderate

This is a good method that is used for switching high currents as the TRIAC can be set to switch on at zero current and it will stay on until the current goes to zero.
So, TRIAC on, then relay on. Before switching off, make sure the TRIAC is enabled, then relay off and after a delay, TRIAC off.
You can get away with a smaller relay as it never switches the high current, all going well, just has to hold it.

 

I can't see any confirmation from the the TS that it is a universal (Series wound motor.) At the momment it seems that it is only an assumption that it is a universal motor. If it is a single phase induction motor I don't think a soft start is possible.

Les.